The multiplexes will fill up with “Best Exotic Marigold Hotel” clones, restaurants will close for the night at 7:30 and the talk-radio station in the national Crown Victoria will always be turned up too loud. As the car itself hums along at 25 mph.

In “What to Expect When No One’s Expecting, “ Jonathan V. Last sounds the alarm on America’s steadily declining birthrate, noting that it has fallen below the 2.1 kids per woman we need just to maintain a flat population and is still heading down, especially among the wealthy and educated. (Women who graduated college are having 1.78 children on average; those with graduate degrees, 1.61. That’s about the same as China’s 1.54, and the Chinese have an official “one child” policy that severely penalizes couples having more than one baby.)

Only immigration and immigrants are keeping the US as a whole growing (though birthrates among immigrants are falling even faster). In 1950, the median age in the US was 30. By 2050, it’ll be 40 — the median age for Florida today.

And what are old people like? Old people are conservative. One manifestation is that they are wary of putting their money in speculative investments, preferring low-risk alternatives such as treasury bills. “So capital pools will shrink,” Last notes. Tomorrow’s Mark Zuckerbergs will have more difficulty reaching a friendly venture capitalist for backing.

And the pool of young innovators will shrink because the number of young will shrink, both absolutely and in relation to the number of oldsters. Virtually all of history’s great Eureka moments have happened to people under 50, with creativity tending to hit a peak among those between 35 and 40.

“In every country, the bulk of entrepreneurs are between 18 and 34 years old,” Last writes. “And in advanced, innovation-driven economies, such as ours, very few entrepreneurs are over the age of 45.”

One scientific study concluded that “With increasing age, people seem to need more time for decision making; furthermore, their choices tend to be more conservative.” That’s worrying in a society that relies on rapid technological change and daring business innovation to generate the wealth that is supposed to pay for all these retirement packages.

Japan, the most rapidly aging society on earth, looks like a snapshot of our future. Its birthrate has plummeted to 1.39 children per woman, despite a major national push to promote families. Its population is on track to shrink from 127 million to 89 million over the next 40 years. Dogs now outnumber children under 10.

Economically, this means an increasingly pyramid-shaped society — with the tip pointing down. Young people will be forced to pay ever-increasing taxes to support the pensions and health care of their parents and grandparents.

An aging society means job growth will be concentrated in old-timer maintenance: audiologist, physical therapist and home-care aides are all listed by the Bureau of Labor Statistics as among the hottest fields. Naturally those jobs will be where the old people are, so the young should forget Brooklyn and think Scottsdale.

Even before all that happens, the new youth austerity — goodbye “Sex and the City,” hello “Girls” — has already begun. Young Americans have been paring back their debt much more quickly than their elders. Only 39% of young adults now hold credit-card debt, down from 50% in 2001.

Yet living standards have declined all the same. The percentage of Americans 18 to 24 who own their home fell from 40% in 2007 to 34% in 2011. Car ownership fell from 73% of young households to 66% in the same period.

Amid all of this belt-tightening, the young still have a higher debt-to-income ratio than their parents.

As Japan’s population shrinks, it faces worries about a “deflationary spiral” of falling wages and prices. (Customers die off, so products don’t sell, so the price gets cut, which leads to the store paying its employees less). That’s also an age issue. For those with savings — such as senior citizens — deflation means more purchasing power. But if you owe, deflation means debts that are harder and harder to pay back.

A hipster in Richard Price’s novel “Lush Life” is asked what he intends to tell his kids about his tattoos. “My kid?” replies the hipster, incredulous. “I’m my own kid.” An entire generation has put off assuming the adult responsibility of raising kids to extend indefinitely its own period of childlike self-absorption. Soon that group will find out the downside of remaining a kid forever: Their parents’ generation will still exert a lot of control over their lives.